Screening Of Yoghurt Starter Cultures For Fermentation

Starter culture of lactic acid bacteria play a crucial role in the quality of yoghurt and superior strain can exert great flavor and taste on yoghurt.It make good sense of screening out superior strain culture for promoting the quality of yoghurt. Twenty-two strains of L.bulgaricus and forty-three strains of S.thermophilus, which were formerly isolated from fermented kunut, goat milk and milk cake in Qinghai, Mongolia, Yunnan and other regions of minorities, were screened for superior starter culture strain.The fermented milk samples were fermented by L. bulgaricus or S.thermophilus or 1:1 ratio with added amount as 5×10 6cfu/g.The fermented milk sample were stored at 4℃when their pH values arrived to 4.5.The fermentation charaiteristics (such as titratable acidity,viscosity,syneresis,free amino nitrogen)of the fermented milk samples when finishing fermentation or under different storage time were checked.The sensory characteristics of these fermented milk samples after stored for 24h were also evaluated.Seven strains of L.bulgaricus with good fermentation and sensory characteristics were screened out: L.bulgaricus, L.bulgaricus B2, L. bulgaricus B4, L.bulgaricus B5, L.bulgaricus B16, L.bulgaricus B1, L.bulgaricus B3 and L.bulgaricus B12. The highest sensory score of the fermented milk sample by these seven strains was 91.9,the lowest was 82.6,and there was no significant differents between the score of 82.6 and L.bulgaricus HB (P>0.05). Visicosity of yoghurt samples fermented by seven strains were above 500cp and yoghurt visicosity of L. bulgaricus B2 could achieved as much as 1084cp after storage for 7d. Changes of titratable acidity ( TA) of yoghurt fermented by L. bulgaricus B2 could raise up to 28.24°T, while TA of L.bulgaricus B12 was only 6.74°T during storage. FAN of yoghurt samples fermented by seven strains were all above 3.00mmol/L, the highest FAN was 8.77 mmol/L.Five strains of S.thermophilus with good fermentation and sensory characteristics were screened out: S.thermophilus S12, S.thermophilus S9, S.thermophilus S6, S.thermophilus S7 and S.thermophilus S21. The highest sensory score of the fermented milk sample by these five strains was 91.9, the lowest was 86.5,and there was no significant differents between the score of 82.6 and S.thermophilus HS (P>0.05). After ripening for 12h, visicosity of yoghurt samples fermented by five strains were all above 1100cp,with the highest visicosity as 1385cp which was fermented by S.thermophilus S12. The syneresis rates of these yoghurt were all above 37.77%.Furthermore, the selected seven L.bulgaricus and five S.thermophilus strains were combined in fermented milk for evaluation their fermentation and sensory characteristics. Two combination ( L.bulgaricus B2 and S.thermophilus S6,L.bulgaricus B16 and S.thermophilus S1) were screened out with favorable fermentation and sensory properties. The sensory scores of these two combination groups were 95.2 and 92.6, TA for 21d storage and visicosity for 28d storage of these two combination groups were 9.99°T and 4.06°T, 1088cp and 1128cp, respectively. The syneresis rates of these two combination groups were 40.90%~43.68% and 40.26%~42.77%, respectively.